Most existing UV light emitting diodes (LEDs) use an electron blocking layer (EBL), which is a ˜15-20 nm, p-doped Mg:AlGaN layer intended to improve hole injection in the active layer as well as to reduce electron leakage current from the active layer. EBLs function quite well for wavelengths >300 nm, but their effectiveness is drastically reduced at shorter wavelengths. This is because at wavelengths <280 nm the barrier layer must carry a high concentration of Al (between 60 and 100%) and the ionization energy of the Mg acceptors is very high. This makes effective p-doping for these wavelengths practically impossible. Therefore the carrier injection is very low and there is the additional problem of unwanted parasitic luminescence. In addition both the internal and external quantum efficiency of LEDs at these wavelengths are extremely low (between 222-240 nm a fraction of a percent and from 250-280 nm. around one percent) and the performance is extremely low. One LED exists at 210 nm with an output power of only 20 nW (Yoshitaka Taniyasu et al., Nature Vol 441, 325ff (2006)).
So far UV LEDs have only been made with conventional p-doped AlGaN EBLs. For deep UV wavelengths this requires high Al concentrations for the EBL whose p-doping is difficult due to the high ionization energy of the magnesium acceptors. Therefore the carrier injection into the active region very poor, leading to poor internal and external quantum efficiencies and, therefore, poor overall performance. The shorter the LED wavelength, the higher the band-gap energy, in turn determining the aluminum content of the AlGaN:Mg EBL layers. Especially for UV-C LEDs (emission from 200 to 280 nm) AlGaN:Mg EBL layers with very high aluminum content are required (typically between 60% and 100%). Due to the high ionization energy of the Mg acceptors (over 500 meV for AlN) the p-doping of AlGaN is very difficult or perhaps impossible (see for example Nam et al., Appl. Phys. Lett. 83, 878 (2003), which is incorporated herein by reference in its entirety). The effectiveness of the AlGaN:Mg EBL at high aluminum content is thereby strongly limited and this approach for UV-C LEDs is completely unusable.
The suppression of parasitic or ancillary luminescence, with the use of an AlN intermediate layer is described in Sumiya et al., “AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxial AlN/Sapphire Templates” Japanese Journal of Applied Physics, 2008, 47(1), 43-46 and Zhang et al., “Suppression of the subband parasitic peak by 1 nm i-AlN interlayer in AlGaN deep ultraviolet light-emitting diodes,” Applied Physics Letters, 2008, 93 (13117-1-3), which are incorporated herein by reference in their entireties. A 1 nm AlN layer was used to remedy the negative effects of the still-present EBL and the wavelengths therefore remained above 260 nm.
In view of the above, it is an object of the present invention to use new layering for increasing the quantum efficiency of LEDs in the deep UV (DUV) region, thereby increasing the overall performance and reducing parasitic luminescence.
It is also an object of the present invention to improve hole injection while reducing electron leakage current without the doping problems that arise when trying to achieve shorter wavelengths.